The invention relates generally to nondestructive testing methods for composite materials, and more particularly to method and system for nondestructive evaluation of composite laminates.
Composite materials, particularly fibrous composite laminates, are of substantial interest to the aerospace industry for aircraft structural components by reason of high strength-to-weight ratios which characterize these materaials. In fabricating composite laminates, numerous performance limiting defects may arise within the laminate structure, including delaminations, porosity, voids, ply gaps and overlaps, resin rich and resin starved regions, fiber misalignment, and cracks. Certain defects not detected in the laminate structure as manufactured may arise in the material during use. Nondestructive testing (NDT) methods are presently used to detect the defects and include x-ray radiography and ultrasonics in various modes such as pulse echo and through transmission. However, the nonhomogeneous anisotropic structure which characterizes fibrous composites severely limits the suitability of conventional methods for nondestructive evaluation of these materials.
The invention eliminates or substantially reduces in critical importance the aforesaid shortcomings in the nondestructive testing art by providing a novel method using leaky Lamb waves to detect and characterize defects in fibrous composite laminates, which defects may not be detectable using conventional NDT techniques. Based on a clear understanding of ultrasonic wave propagation in composite laminate materials, the method of the invention allows detection of defects utilizing plate (Lamb) waves in fluid-loaded, fibrous composite materials. In practicing the method of the invention, leaky Lamb waves are excited in water coupled composite laminate plates by ultrasonic insonification at appropriate angles and frequencies using a two-transducer, pitch-catch geometry. The method takes advantage of the physical distortion in the reflected field, which occurs only at discrete leaky wave conditions, to enhance signal-to-noise ratios of ultrasonic energy reflected from a composite plate. Insertion and extraction of leaky Lamb waves in composites is done by immersing the transducers and plate in a sound coupling fluid (water) or by using two captured water columns which couple the transducers and plate.
The invention is therefore useful for nondestructive evaluation of substantially any plate material, and is of particular utility for the detection and characterization of porosity and other defects in structural composite laminates. The method of the invention has clear superiority over conventional techniques, such as the pulse echo technique, in the detection of dilute porosity, and may find broad application in manufacturing quality assurance inspections.
It is, therefore, a principle object of the invention to provide novel nondestructive evaluation method and system.
It is a further object of the invention to provide method and system for nondestructively testing composite materials.
It is another object of the invention to provide nondestructive testing method and system utilizing leaky Lamb waves.
These and other objects of the invention will become apparent as the detailed description of representative embodiments proceeds.